1. Technical Field
The disclosure generally relates to illumination devices, and particularly, to an illumination device with anti-glare function.
2. Description of Related Art
Light emitting diodes (LEDs) are extensively used as light sources due to their high luminous efficiency, low power consumption, and long lifespan. Although the LEDs can emit bright light to illuminate a dark environment, a glare may occur when bright light from the LEDs directly radiates into the eyes. For example, as shown in FIG. 8, in a typical application of LEDs 101, the LEDs 101 are arranged on a ceiling to provide overhead lighting. Because the LEDs 101 emit light radially, the user with an elevation angle with respect to the LEDs 101 in a range from about 45 degrees to about 85 degrees may see glares from the LEDs 101. The glare causes eye strain and fatigue, which may lead to serious headaches and other discomforts.
Referring to FIG. 9, a road lamp 201 is used for lighting a road to achieve an illumination range around a center of road lamp 201. The illumination range along an X-direction is greater than that along a Y-direction. The X-direction is perpendicular to the Y-direction as shown, and the X-direction expresses the extending direction of the road 11. The distribution curve of the road lamp 201 in the X-direction is shown in FIG. 10; as shown, point A corresponds to 50% of the maximal light intensity of the road lamp 201 in a range from 0° to 90° from a downward vertical line. Point B corresponds to the maximal light intensity of the road lamp 201 in a range from 0° to 90° from the downward vertical line. It can be seen that the angle θ between the light which has 50% maximal light intensity of the light of the road lamp 201 and the downward vertical line is used to characterize radiation range of the road lamp 201. However, the radiation range in the extending direction of the road distributes symmetrically respect to the center of road lamp 201. Specifically, the radiation range is consisted of a first angular range and an opposite second angular range located in the X-direction. The first angular range is directed at an angle θ1 between the light with 50% maximal light intensity of the light and the downward vertical line toward a part of illumination area. The second angular range is directed at an angle θ2 between the light with 50% maximal light intensity of the light and the downward vertical line toward the other part of illumination area. The angles θ1, θ2 are equivalent, usually θ1=θ2=75°, resulting in glare to the drivers on the road.
Therefore, there is a desire to provide an illumination device that overcomes the described limitations.